(1) Field of the Invention
The present invention relates generally to a calender for a sheet of paper, and more particularly to a calender which performs a surface process on paper after it is dried by a drier, to make it smooth and glossy.
(2) Description of the Related Art
In paper mills, a layer of paper made by a paper-making section is pressed to remove water by a press. Then, the paper is heated and dried. Next, a calender is employed as a machine in which paper is pressed by rollers to glaze or smooth it.
Typical examples of calenders are a chilled nip calender, a soft nip calender, and a shoe calender. The chilled nip calender is equipped with chilled metal rolls to form at least one pair of nips. The soft nip calender is constructed of a metal roll and an elastic resin roll. In the soft nip calender, only a pair of nips is formed on the periphery of the resin roll. The shoe calender is constructed of a metal roll, a tube sleeve disposed opposite the metal roll and a shoe which is inside of the tube sleeve. The shoe is pressed against the inner periphery of the sleeve to form a nip.
Since the present invention relates to the above-described shoe calender, two conventional shoe calenders will hereinafter be described with reference to FIGS. 13 to 17.
FIGS. 13 and 14 show a first conventional shoe calender described in patent reference 1. The conventional shoe calender is constructed of an upper half part including a metal roll 10, and a lower half part including a cylindrical stationary beam 1, a sleeve 2, etc. The cylindrical stationary beam 1 is fixedly attached to a support leg 11. The outer periphery of the stationary beam 1 is provided with guide members 8 at suitable intervals with respect to the center of the stationary beam 1.
A sleeve 12 is provided to cover the cylindrical stationary beam 1 and rotatably supported by the guide members 8. The opposite ends of the sleeve 12 are further supported by clamp discs 9 to make the interior airtight.
In the conventional shoe calender constructed as described above, when a paper sheet 15 is calendered, the lower half part of the calender with the sleeve 2 is brought into contact with the peripheral surface of the metal roll 10 through the paper sheet 15, as shown in FIG. 13. The sleeve 2 is pressurized by applying pressurized oil to the pressurizing shoe 3 and utilizing the deformation of the sleeve 2 that develops when the sleeve 2 is pressed radially outward.
The stationary beam 1 is further provided with lubricating-oil supply passages 4, 5 and lubricating-oil collection passages 6, 7. The first lubricating-oil supply passage 4 is connected to the lower portion of the pressurizing shoe 3 so that pressurizing force is applied to the pressurizing shoe 3. The second lubricating-oil supply passage 5 is opened at the outer periphery of the stationary beam 1 so that lubricating oil can be supplied to the inner periphery of the sleeve 2.
FIGS. 15 through 17 show a second conventional shoe calender described in patent reference 2. The conventional shoe calender is basically the same in construction as the first conventional shoe calender shown in FIGS. 13 and 14. As in the first conventional shoe calender, a flexible jacket 32 is pressed against a metal roll 10 to calender a paper sheet 15.
That is, to calender the paper sheet 15, a shoe roll 30 is pressed against the metal roll 10 by a pressurizing shoe 18 provided inside the flexible jacket 32. Reference numeral 95 denotes a pressurizing unit for the metal roll 10. Reference numeral 34 denotes a support beam for the pressurizing shoe 18, and 20 denotes a pressurizing unit.
As shown in FIG. 16, the opposite ends of the flexible jacket 32 are fixed to end plates 24, 26. If the pressurizing unit 20 is actuated, the pressurizing shoe 18 projects in the radial direction of the flexible jacket 32 and deforms the flexible jacket 32. As a result, the paper sheet 15 is pressurized between the metal roll 10 and the flexible jacket 32.
In the conventional shoe calender shown in FIGS. 13 and 14, the sleeve 2 is rotated by the rotational force of the metal roll 10 which is rotated by a driving unit (not shown). Because of this, if the pressurizing force of the pressurizing shoe 3 is weak, the transmission of the rotational force will be insufficient, and consequently, the sleeve 2 will slip easily. Conversely, if it is strong, the friction between the pressurizing shoe 3 and the sleeve 2 will increase. As a result, heat will be generated and the sleeve 2 will be elliptically deformed.
Hence, the shoe calender is provided with the lubricating-oil supply passages 5, and lubricating oil is supplied to the inner periphery of the sleeve 2 to prevent generation of heat and perform lubrication. In addition, the guide members 8 are disposed inside the sleeve 2 to prevent deformation of the sleeve 2.
However, if the pressurizing force reaches a predetermined value or greater, deformation of the sleeve 2 will become great and therefore gaps will be produced between the guide member 8 and the sleeve 2. As a result the effect of the guide members 8 will no longer be obtained.
Because of the gaps between the guide members 8 and the sleeve 2, the sleeve 2 is insufficiently supported and therefore vibrates. As a result, there is a problem that because of the vibration, the trace of vibration will occur in the paper sheet 15.
In chilled nip calenders, incidentally, paper is passed between rolls in contact with each other. However, in soft nip calenders, if a rubber roll is contacted with a high-temperature metal roll without paper, the rubber will degrade. Because of this, the rubber roll is held away from the metal roll until paper is passed through. After paper is passed through, the rubber roll is pressed against the metal roll through the paper.
On the other hand, in the conventional shoe calender (patent reference 1), the sleeve 2 is of a driven type. That is, the sleeve 2 is rotated by contacting with the metal roll 10. In this shoe calender, as with chilled nip calenders, paper is passed between the sleeve 2 and the metal roll 10 after the sleeve 2 is contacted with the metal roll 10. Because of this, before paper is passed through, the outer periphery of the sleeve 2 is contacted directly with the high-temperature metal roll 10.
However, since the outer periphery of the sleeve 2 of the shoe calender is constructed of elastic synthetic resin, if the sleeve 2 of the shoe calender is exposed to high temperature for along time and rises in temperature, then the quality will degrade and the life will be shortened. Particularly, in such a shoe calender, the nip passage time is long and therefore the contact area (i.e., contact time) between the outer periphery of the sleeve 2 and the metal roll 10 is long. As a result, the temperature of the outer periphery of the sleeve 2 becomes considerably high.
To prevent the problem of high temperature, it is contemplated that the sleeve 2 is held away from the metal roll 10 until paper is passed through. In soft nip calenders, such a process is often performed. However, since the sleeve 2 in the conventional shoe calender (patent reference 1) has no driving unit, the sleeve 2 will no longer rotate if it is moved away from the metal roll 10. Therefore, in the case where the sleeve 2 is contacted with the metal roll 10 after paper is passed through, it is necessary to contact the sleeve 2 with the metal roll 10 being rotated. In such a case, paper is broken as soon as the sleeve 2 not being rotated is contacted with paper.
Therefore, in the conventional shoe calender, the sleeve 2 must be held in contact with the metal roll 10 during operation. As a result, the outer periphery of the sleeve 2, which is constructed of a material whose heat-resisting temperature is low (e.g., polyurethane), will reach a considerably high temperature and degrade quickly.
In the conventional shoe calender shown in FIG. 17, the flexible jacket 32 can rotate. That is, the end plate 24 or 26 is driven by a driving unit (not shown). A gear 56 is rotated by a driving shaft 48. In this way, the flexible jacket 32 is rotated. Since the moving speed of the paper sheet 15 can be synchronized with the rotational speed of the flexible jacket 32, breaking of the paper sheet 15 can be reduced.
However, as with the conventional shoe calender shown in FIGS. 13 and 14, the pressurizing shoe 18 contacts with the metal roll 10 at one point on the flexible jacket 32. Therefore, in combination with centrifugal force, etc., the flexible jacket 32 is elliptically deformed when it rotates.
That is, since the flexible jacket 32 is supported only at the position of the pressurizing shoe 18, deformation of the flexible jacket 32 becomes great and it rotates elliptically. Because of this, vibration is generated by rotation and the runout of the jacket 32 occurs. Thus, the calender cannot be operated at a high speed.